Using platelet integrin alphaIIb-beta3, the fibrinogen receptor, as a model, it has been shown that the cytoplasmic domain of the integrin plays a key role in the process of inside-out signaling. Using the yeast two-hybrid system, the applicant and his co-workers have isolated a novel protein of ~22 kD that specifically interacts with the cytoplasmic domain of alphaIIb. The protein has two EF hand motifs (putative Ca++ binding domains) and named Ca++ and integrin-binding (CIB) protein. This protein has significant homology to calcineurin B and calmodulin, which are well characterized calcium dependent regulatory proteins. They have expressed recombinant CIB in bacteria as a GST-fusion protein and are now in the process of generating monoclonal and polyclonal antibodies. Specific aims of the project are: 1) to further characterize the interaction of CIB with other proteins and other integrins by immunoprecipitation, the yeast two-hybrid system and other techniques; 2) to determine the specific site of interaction in both the integrin and the CIB by site directed mutagenesis and other techniques; 3) to determine the functional role of this protein by over-expression in cell lines, dominant negative mutants, and antisense technology.